The present invention relates to oil and gas drilling, and more particularly to methods for forming a wellbore. In many drilling operations a drilling fluid is used to clear the material cut by the tool string bit from the wellbore and maintains a substantial hydrostatic pressure at the depth of the tool string bit that withstands the pressure produced in the surrounding formation. However, this conventional drilling is slowed by the confining pressure exerted by a column of mud in the wellbore. The pressure at the bottom of the wellbore is typically kept at a pressure greater than or equal to the pressure of the fluid pressure in the formation being drilled. The confining pressure of the mud increases the strength and plasticity of rock, reducing the efficiency of indentation and shear cutting.
Another known method for drilling a wellbore is known as Underbalanced Drilling, or UBD. During an UBD procedure, the pressure in the wellbore is maintained lower than the fluid pressure in the formation being drilled. It is believed that using UBD has several advantages over conventional drilling. One advantage is that it is easier to break up the formation being drilled and thereby drilling at an increased Rate of Penetration (ROP). However, UBD may reduce the stability of the wellbore, thereby increasing the risk of the wellbore collapsing in on itself because of the low pressure over the entire open section of the hole. Thus, it may be beneficial to create and maintain a low pressure at the bottom of the wellbore while maintaining a normal pressure higher up the wellbore.
The prior art has addressed drilling operations in an UBD environment. Such issues have been addressed in the U.S. Pat. No. 6,237,701 to Kollé et al., which is herein incorporated by reference for all that it contains. The '701 patent discloses suction pressure pulses generated within a borehole by closing a valve that interrupts the flow of a drilling fluid (e.g., drilling mud) circulating through one or more high velocity flow courses within the borehole. In one embodiment in which the suction pressure pulses are applied to improve the efficiency of a drilling bit, the valve interrupts the flow of drilling mud directed through the bit and thus through high velocity flow course(s) disposed downstream of the bit. Arresting flow of the drilling mud through the high velocity flow course(s) generates suction pressure pulses of substantial magnitude over a face of the tool string bit. The suction pressure pulses provide a sufficient differential pressure that weakens the rock through which the tool string bit is advancing and also increase the force with which the tool string bit is being advanced toward the rock at the bottom of the borehole. However, the flow of drilling mud into an inlet port of the valve is not interrupted, so that fluid motors can still be used to rotate the tool string bit. When the valve is closed, the drilling mud continues to flow into the valve and subsequently flows back into the borehole. The suction pressure pulses can also be applied to a short section of the borehole wall to produce seismic pulses, or to provide remediation of formation damage (by drawing fines from the wall of a borehole to enhance oil and gas production rates), or can be employed for descaling tubes within a borehole.
U.S. Pat. No. 5,740,127 to Van Steenwyk, et al., which is herein incorporated by reference for all that it contains, discloses a fluid pulsing apparatus operable in a drill pipe in a well in which well drilling fluid flows, wherein pressure pulses are created by restricting one or more of several hydraulically parallel paths, constant working pressure regulating valves with a long time constant relative to the transient pulses are constructed in the hydraulically parallel paths. The valves operate to produce a more consistent pulse character allowing production of pulses at low flow rates of drilling fluid that are of sufficient amplitude to be more easily detected on the Earth's surface and restriction of amplitude of pressure pulses at high flow rates of drilling fluid to limit equipment damage and loss of hydraulic energy. The valves function by varying the flowing cross sectional area of the hydraulically parallel paths.